[FE] Allow using sealed modifier on interface and compute sealed modality for them

#KT-20423

compiler/testData/diagnostics/tests/sealed/interfaces/sealedInterfacesDisabled.txt

@@ -0,0 +1,7 @@
+package
+
+public sealed interface Base {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}